Now that we have Megalosaurus and Kaprosuchus, taming Rays has never been easier. You can pick them up with either animal on PvP OR PvE (it works, trust me). Just latch on and hold it there while a tribemate feeds it when needed, or if you're alone carefully pull it into whatever trap you may have prepared. The biggest problem with this is making sure you don't lift the Ray out of the water or it will instantly die, but as long as you're careful the process is quick and easy
They have terrible turning circle, approach one from above and feed it. Then let it swim circles around you. The occasional time it gets close you can predict its attacks, it “bucks” just before the attack, which is the tail swinging underneath. Quick swim up or back avoids it. Be prepared to get dizzy, long time between feeding
Tame a kaprosuchos and swim to one of these bad boys and do it's alternate attack it will grab it in it's mouth. walk onto the shore where its safe and have another person feed it while in the gators mouth (if you unmount it drops it) it wont attack or die because it's technically not on the shore.